Preparations have been made for a study of mixing in the left ventricle of the dog heart. Thermistor probes have been designed around 18 gage hypodermic tubing. Three of these probes are inserted through the myocardium so that they are spaced 3/4 inches apart and lie in a single plane passing through the major axis of the ventricle. Each probe contains three thermistors which can be extended from protected recesses after passing through the heart wall. The thermistors have time constants of about 40 milliseconds. Cool saline is injected through a #8 NIH catheter inserted in the ventricle. The changes in temperature that take place at the nine ventricular sites are stored with a digital tape recorder via a minicomputer facility. The variability of the deltaT at end diastole will be used as a measure of the completeness of ventricular mixing. The differences in the degree of mixing from dog to dog and from beat to beat and the effect of indicator catheter tip location, heart rate and stroke volume on mixing will be determined. Comparable information will be collected on a hydraulically operated artificial ventricle that will be used in the development of instrumentation and procedures for measuring ventricular volume by thermodilution. The goal of the project is to produce a useful thermodilution method.